U.S. Pat. No. 4,520,262 discloses a light barrier system utilizing a plurality of individual transmitters of light, a like plurality of individual receivers of light, and a like plurality of light conducting paths. Each transmitter is connected to its corresponding receiver by a corresponding path so that the light emitted by any transmitter can only impinge upon the corresponding receiver. The light emitted is in the form of pulses. The transmitters are selectively actuated and deactuated in selected format by a multiplexer. The corresponding receivers are synchronized to respond in the same format by a demultiplexer. When the system functions properly each receiver will receive and respond only to the light pulse emitted by the corresponding transmitter. A large circuit is used to check the response of each receiver when its transmitter emits a light pulse and also checks this response when its transmitter does not emit a light pulse. Consequently faulty operation of the receiver either by not receiving a light pulse when its transmitter produces a pulse or by receiving a spurious pulse when its transmitter does not produce a pulse can be detected and appropriate corrective action can be taken.
However, this known system suffers from the disadvantage that the logic circuit does not always respond reliably to all types of faults and particularly to faults created by failures of individual components. This circuit is also subject to false alarms triggered when the phase position and the pulse widths of the individual pulses do not coincide with the high degree of accuracy required.